The present invention relates to a field of a high-frequency heating apparatus for conducting a dielectric heating by using a magnetron such as electronic ranges, or for conducting an induction heating by using an induction heating coil such as electromagnetic cooking devices.
Conventionally, for this kind of inverter circuit, the down sizing, weight reduction, and cost reduction are positively promoted in various fields. Further, also in the high-frequency heating apparatus to cook the food by the microwave generated by the magnetron, the down sizing and weight reduction of the power source to drive the magnetron are required, and the requirement is realized by employing the switching technology.
Further, in order to reduce a switching loss of the switching element which operates at the high frequency, a system using a resonance type circuit system, which is an important technology of the switching power source, is also realized. Further, prior to the present invention, a problem in which, by the action of the resonance circuit, the voltage applied onto the switching element is increased, thereby, the withstand voltage of the switching element or relating electric parts is increased, and as the result, the size and the cost are also increased, has been solved by the structure shown below.
That is, as shown in FIG. 10, the system comprises: a DC power source 1; a leakage transformer 2 connected to the DC power source; the first switching element 6 serially connected to a primary winding 3 side of the leakage transformer 2; the first capacitor 4; a serial circuit of the second capacitor 5 and the second switching element 7; a drive means 8 having an oscillator to drive the first switching element 6 and the second switching element 7; a rectifier means 10 connected to the secondary winding 9 side of the leakage transformer 2; and a magnetron 11 connected to the rectifier means 10, and the system is structured such that the serial circuit of the first capacitor 5 and the second switching element 7 is connected in parallel with the primary winding 3 side of the leakage transformer 2.
The characteristic of the circuit structure is a point in which, by using the second auxiliary capacitor 5 having a larger capacity value than the first capacitor 4 constituting the resonance circuit together with the leakage transformer 2, the applied voltage of the first main switching element 6 can be reduced.
However, in the conventional inverter circuit, because the second switching element 7 operates at the higher voltage different from that of the first switching element 6, the drive circuit to dive it requires a high withstand voltage drive circuit such as an insulation type. Accordingly, it becomes a major interference for the simplification of the circuit.
In order to solve the above problems, the present invention comprises: an inductive element whose one end is connected to the DC power source; the first switching element to control the supplying status of the DC power source to the inductive element; the first and second capacitors for the resonance of the inductive element; and the second switching element serially connected to the second capacitor, wherein the second capacitor is serially connected to the first switching element, and the drive signal generating means is connected between the first switching element side of the second capacitor and the control terminal of the second switching element.
According to a first aspect of the invention, the applied voltage of the first switching element can be decreased, and the second switching element can be driven by the very simple structure.
Further, according to a second aspect of the invention, by the negative voltage limiting circuit, because the negative excessive voltage is blocked, the resistance value of the second drive section is set to a small value, and the drive signal of the second switching element is increased, thereby, its on-loss can be reduced.
Further, according to a third aspect of the invention, by the positive voltage limiting circuit, because the positive voltage can be limited, the resistance value of the second drive section is set to smaller value, and the drive signal of the second switching element just before the completion of the mode 4, can be increased, thereby, its on-loss can be reduced.
Further, according to a fourth aspect of the invention, when this resistance value is structured so that it is specifically decreased during a period in which the potential of the common connection point is lower than the potential of the DC power source, the off-operation of the second switching element is quickened, thereby, the switching loss can be reduced.